1. Field of the Invention
This invention relates to improvements in syringes for effecting subcutaneous, intravenous, intramuscular and rectal injections of medicaments and the like into human beings and animals and for taking blood samples therefrom.
More particularly, the syringe of the invention is of the type in which only a small part of the apparatus is discarded after each use, the rest of the syringe being used repeatedly.
2. Description of the Prior Art
It is well known that most syringes now used in hospitals, offices and health centers are of the disposable type, i.e. are disposed of or discarded after one use. These syringes usually comprise two pieces: a barrel and a plunger mounted for reciprocable movement in the barrel, both parts usually being made of plastic material.
As a disposable product, a syringe of this type, although made by modern mass production methods such as injection moulding, is relatively expensive due to the materials and precision involved in the manufacture thereof.
It has been proposed to combine a barrel and a plunger with a replaceable container positioned at the discharge end of the barrel, the container being introverted or collapsed upon itself in use by action of the plunger to eject or administer the medicament contained in the container. A syringe of this type is diclosed in U.S. patent application Ser. No. 684,020 (Nils Billy Nilson) filed May 7, 1976 (now abandoned).
In the syringe disclosed in said patent application the container forms the end wall of the barrel and comprises a substantially rigid first wall portion at the exterior side of the end wall and a flexible second wall portion at the interior side of the end wall and introvertible upon the inside surface of the first wall portion. A nozzle is provided on said first wall portion for attaching a hypodermic needle thereto.
In the embodiment of this prior art syringe as described in said patent application the container is formed as a spherical bulb having substantially the same diameter as the cylinder space formed by the barrel. When an empty container is attached to the barrel said flexible second wall portion being introverted upon the inside surface of said first wall portion, and the container is to be filled, the piston is withdrawn and the flexible wall portion is carried along by the piston due to subatomospheric pressure created between the piston and the flexible wall portion while liquid such as blood or a medicament is being sucked into the container. However, it has been found that a substantial subatmospheric pressure (vacuum) will be created by the piston during the later part of the displacement thereof, which requires a considerable force to be applied to the piston at the end of the stroke necessary for filling the container. Under circumstances the container cannot even be completely filled. Moreover, the force has to be maintained on the piston; if the piston is released it will spring back and part of the liquid in the container will be expelled. This means that in case of a medicament to be administered the portioning thereof will be inaccurate and insufficient, and that in case of a blood sample to be taken, hazardous reinjection of the blood into the vein will take place.
Said drawback of the prior art syringe also presents itself at the safety step always performed in the administration of a medicament into a human being when the cannula has been put in, to make sure that the cannula is in the proper position for the specific administration to be performed. This safety step commonly called aspiration is performed by first ejecting a minimum quantity of the medicament from the container before the cannula is put in and then, when the cannula has been put in, withdraw the piston slightly. If blood enters the container the cannula has pierced a vein and if not, the cannula is inserted into the tissues. Thus, it can be checked if the cannula is in proper position for an intravenous or an intramuscular or a subcutaneous injection. Due to the vacuum created in the prior art syringe according to said patent application it is difficult to make the said check at the administration even if the container used is a prefilled container.
A problem of the syringe described in said patent application may also be seen in the difficulty of completely emptying the container by actuating the plunger to eject or administer the medicament contained in the container, due to the fact that the container is not completely collapsed when the plunger bottoms therein. This is a serious drawback in many cases wherein the medicament comprises a small volume which has to be administered with great accuracy.